AI denoiser removing detail
AI Denoisers use algorithms to guess which pixels are fireflies, and what colour that pixel was supposed to be. The algorithms are limited to the data that it can collect from the Denoising Data and the surrounding pixels. Thus, if insufficient data has been passed to it (not enough samples), the denoiser will create a "washed out" effect where you're losing detail. In truth, the denoiser will almost always cause loss of detail, it's just that when the sampling is high enough, the loss of detail is negligible.

You haven't done anything wrong with the compositor nodes, you're going to have to increase the samples in this case. 

If you're worried about render times, check out this video by BlenderGuru. It was created pre-2.80, but much of the content is still relevant.